Aerial work platforms provide access to elevated areas so that workers may easily and safely perform routine maintenance on elevated fixtures, or gain access to elevated construction or storage areas. Generally, a scissors-type aerial work platform consists of a work platform, a scissors lift, and a support base. The scissors lift is extended vertically, usually by a hydraulic actuator mounted on the support base, to raise the work platform to a desired height. The support base may be a mobile structure, such as a small cart or a truck, or a stationary structure.
A scissors lift is a series of pivotally connected scissors-type linkages. Each scissors linkage is formed by pivotally connecting rectangular cross-section elongate beams ("rectangular link tubes") at central axes. Typically, two of the rectangular link tubes are welded together so as to form an "inner link weldment." Another pair of rectangular link tubes are connected together and extend outside the inner link weldment. These two rectangular link tubes form an "outer link weldment." The connected inner and outer link weldments are called a "stack." The outer and inner link weldments are connected at their centers and are rotatable to a first formation in which the two link weldments form an "X," and a second formation in which the two link weldments extend substantially along one another. The stack is connected in series to another stack by pivotally connecting the lower end pair of the outer and inner link weldments to the upper end pair of the outer and inner link weldments of another stack. Additional stacks can be added to form a scissors lift of a desirable size. The uppermost stack is connected to a work platform, with one of the inner and outer link weldments pivotally connected, usually by a hinge, and the other end slidably attached to the work platform. The lowermost stack is similarly connected to the support base.
Typically, at each pivotal connection of the link weldments, each rectangular link tube has a cross tube welded into a hole within the link tube, the cross tube positioned perpendicular to the beam's longitudinal axis. The rectangular link tubes of the inner link weldment are rigidly connected by a central link cross tube. Shorter cross tubes extend through holes in the rectangular link tubes of the outer link weldment. A pivot pin extends through the cross tubes.
Applying a force to one beam of a scissors linkage causes the inner and outer link weldments to rotate relative to one another about their central axes. This rotation displaces the ends of the inner and outer link weldments, which are pivotally connected to the inner and outer link weldments in another stack. The inner and outer link weldments of the adjacent stack also rotate relative to one another. Thus, applying a force to at least one stack in the scissors lift transfers the force to the entire scissors lift structure. As a result, each stack extends or retracts, which in turn elevates or lowers the work platform.
A common problem in scissors-type lifting devices is that the adjoining rectangular link tubes of the inner and outer link weldments undergo opposite moments of torque as they rotate about the pivot connections. This loading fatigues the weld joints between the cross tubes and the rectangular link tubes. Fatigue is most severe at the inside weld joints on the facing surfaces of the rectangular link tubes. At this location, a weld root is formed between the outer surface of the cross tubes and the inner surface of the cross tube hole in the rectangular link tubes. Repetitive loading eventually results in fatigue fractures along these weld roots after 20,000-30,000 operational cycles. As the fractures significantly degrade the structural integrity of the scissors lift, extensive repairs or replacement of parts must be accomplished before the scissors lift can be safely used again.
As will be readily appreciated from the foregoing discussion, there is a need for a cross tube section that reduces the transfer of the torque loading force to the weld joints between the cross tube section and the rectangular link tube, which in turn reduces metal fatigue and thus increases the useful life of the scissors lift apparatus. The present invention is directed to fulfilling this need.